Due to space and mass limitations, spacecraft used in extended space missions must generally either generate their own power, e.g., through solar collectors, or use nuclear power. Solar powered spacecraft typically use a number of large area solar panels that are deployed once the spacecraft is in flight. Each panel generally includes a two-dimensional array of solar cells wherein each solar cell has an active area for converting sunlight to electricity and a pair of electrodes, e.g., a front electrode and a back electrode. A number of such cells may be arranged in electrical series in a "string", for example, by connecting a positive electrode of one cell (e.g, the front electrode) to the negative electrode (e.g, back electrode) of an adjacent cell. By connecting cells in such a string, an increased voltage can be achieved for the spacecraft electrical system. A number of these strings can be connected in parallel to provide an increased current.
Such solar power systems have proved to be highly reliable for many missions and have the ability to generate power indefinitely. However, solar power systems are problematic for deep space missions such as planetary probes that venture beyond Mars' orbit or proposed probes for approaching comets in highly elliptical orbits. In particular, the system parameters of voltage and current can vary dramatically in such missions. The current generated by a solar cell is dependent upon the solar intensity incident on the cell. Accordingly, the current produced by a given cell or string of cells varies in proportion to the square of the distance from the sun, and reaches significantly reduced levels, relative to near earth currents, when a spacecraft passes beyond the Mars orbit. On the other hand, as temperature decreases, solar cell voltage increases. Consequently, the voltage of a given cell or string of cells increases with increasing distance from the sun.
Because of these variations, solar power has not been viewed as a viable option for certain deep space missions. Such missions have therefore been designed for nuclear power. However, nuclear power systems have become problematic due to perceived safety issues and political complications.
There have been some proposals for reconfigurable solar systems, i.e., systems where solar cells or cell groups can alternatively be arranged in a series or a parallel configuration. However, these proposals generally either i) have been complicated in terms of non-generator elements (e.g., switches, sensors, etc.) required, thereby increasing cost, size and mass; or, ii) have provided only a very limited ability to vary system parameters through reconfiguration. As a result, it is apparent that such proposals have not fully addressed the needs of a range of deep space missions.